Use amd64_target_description to get tdesc_amd64

Use amd64_target_description to get tdesc_amd64

This patch changes amd64-*-tdep.c files to use function
amd64_target_description to get the right target description rather than
use the variable tdesd_amd64.

gdb:

2017-09-04  Yao Qi  <yao.qi@linaro.org>

	* amd64-darwin-tdep.c: Include "x86-xstate.h".
	(x86_darwin_init_abi_64): Call amd64_target_description.
	* amd64-dicos-tdep.c: Likewise.
	* amd64-fbsd-nat.c: Likewise.
	* amd64-fbsd-tdep.c: Likewise.
	* amd64-nbsd-tdep.c: Likewise.
	* amd64-obsd-tdep.c: Likewise.
	* amd64-sol2-tdep.c: Likewise.
	* amd64-windows-tdep.c: Likewise.
	* amd64-tdep.h (tdesc_amd64): Remove the declaration.